DESCRIPTION (Taken directly from the application) Despite profound implications for gene therapy of cystic fibrosis (CF), we are unsure which cell lineage model applies at the important small and large airway sites of CF lung disease progression. In model 1, which is pertinent to the hematopoietic system and epidermis, stem cells are relatively rare and most proliferation occurs in a transiently amplifying compartment which then generates terminal cells. In model 2, which theoretically may apply to the airway epithelium, there are no true stem cells in the classical sense but many of the non-terminal cells have great plasticity and are capable of extensive growth and differentiation when needed. The consequences for gene therapy are that in "1" a permanent cure depends upon transfection of relatively few stem cells while in "2" it is more important to target many cells. It is of immediate practical importance to identify and determine the topographic distribution of airway epithelial cells that have an extensive progenitorial capacity. To accomplish this, the following specific aims are proposed: 1) To test the hypothesis that proximal airway epithelial stem cells are confined to a distinct morphological compartment. 2) To determine which theoretical cell lineage model best applies to the pseudostratified airway epithelium of humans. 3) To test the hypothesis that there is a subcompartment of non-ciliated bronchiolar cells (Clara cells) that serve as stem cells. Specific subpopulations of airway epithelial cells from both transgenic animals and humans will be isolated and their proliferative capacity and differentiation potential will be determined. The proposed studies will establish the relationship between classic morphological cell type, expression of specific markers, proliferative capacity and differentiation potential. By rigorously testing the stem cell properties of specific airway epithelial cells we will develop a model for cell lineages and identify the most appropriate target cells for gene therapy of CF in the airways.